Method and apparatus for selectively exciting a matrix of voltage responsive devices

ABSTRACT

A method and apparatus for selectively exciting a matrix of voltage responsive devices in which each of said device is responsive to a voltage difference between a row and column drive line. A plurality of row and column drive lines are fed with an AC voltage, the phases of said voltage on said column drive lines being different from the phases on said row drive lines. A device is selectively excited by adjusting the phase of voltage on the row and column drive lines, associated with said selected device, to be equal to each other, and different from either the phases on other column or other row drive lines. The three different phases of voltage available (one on unselected columns, another on unselected rows, the third on the selected row and column) differ in increments of 2 pi /3. In this manner the voltage across a selected device can be reduced to zero without affecting the amplitude of voltage applied to any other device.

Sta 1 Freiser et al.

1 1 METHOD AND APPARATUS FOR SELECTIVELY EXCITING A MATRIX OF VOLTAGE RESPONSIVE DEVICES [75] Inventors: Marvin J. Freiser, Scarborough;

Dale T. Teaney. Croton-on-Hudson. both of NY.

[73] Assignee: International Business Machines Corporation, Armonk. NY.

[22] Filed: June 7, I974 [21] Appl. No.: 477,204

[52] US. Cl... 340/324 M; 315/169 R; 340/166 EL;

3.466.501 9/1969 Young 315/169 R 3.654.606 4/1972 Marlowe et a1.... 340/166 EL 3740,717 6/1973 Huener et a1. 340/336 3.835.463 9/1974 Tsukamoto et a1. 340/324 M 3.839.715 10/1974 Andoh et a1. 340/324 M OTHER PUBLICATIONS Plasma Display Phase Select. P.E. Oberg and G. F.

rjiiilii'; its;

J if 51 July 15, 1975 Saunter. Information Display, March/April 1969, pp. 3537.

Primary Examiner-David L. Trafton Atmrney, Agent, or FirmP0llock, Philpitt & Vandesande 57 ABSTRACT A method and apparatus for selectively exciting a matrix of voltage responsive devices in which each of said device is responsive to a voltage difference between a row and column drive line. A plurality of row and column drive lines are fed with an AC voltage. the phases of said voltage on said column drive lines being different from the phases on said row drive lines. A device is selectively excited by adjusting the phase of voltage on the row and column drive lines, associated with said selected device, to be equal to each other. and different from either the phases on other column or other row drive lines. The three different phases of voltage available (one on unselected columns. another on unselected rows. the third on the selected row and column) differ in increments of 21r/3. In this manner the voltage across a selected device can be reduced to zero without affecting the amplitude of voltage applied to any other device.

16 Claims, 18 Drawing Figures QR mayo/saw PATEHTEDJUU 5 I975 3,895, 373

52 f 55 PUSH JJZVH/O VOLTAGE PULL GENERATOR AMP 55 54 FIG.8 VH/F W 41 f fL 42 VOLTAGE GENERATOR PHASE /45 45 SHIFT 46 PHASE 44 SHIFT ,3 -21 5 55 53 44 42 j J A a FIG. 10b

METHOD AND APPARATUS FOR SELECTIVELY EXCITING A MATRIX OF VOLTAGE RESPONSIVE DEVICES One application of the foregoing is in driving a matrix of liquid crystals comprising an optical display. In such application a nematic liquid crystal located between orthogonal polarizers can be rendered opaque by the application of a voltage and light transmissive by selective removal of said voltage. Since there is a delay in the response of the liquid crystal between the time the voltage is applied and the time the crystal becomes opaque the signals driving different rows of the matrix can be displaced in time, i.e., multiplexed.

A further embodiment provides a further driving voltage of higher frequency to the row and column drive lines for decreasing the delay in the opaque to transmissive transition and for increasing the delay in the transmissive to opaque transition. The columns are normally not driven by this further voltage and the rows are all driven by a further voltage of equal amplitude and phase. To select a particular device the column drive line associated therewith has applied to it a further voltage twice the amplitude with the same phase as the row driving voltage and the row driving voltage associated with the selected device is altered by 11' radians. In this manner the further voltage component across the selected device is three times the further voltage component applied to each other device. The use of the two driving voltages of different frequencies allows larger matrices to be effectively driven.

Other embodiments include different combinations of transmissive and opaque states to which the devices are driven. Furthermore, the devices need not comprise nematic liquid crystals but may be other voltage responsive devices.

BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for selectively exciting a selected voltage responsive device in a matrix of voltage responsive devices where it is necessary to alter the voltage applied to a particular selected device without varying the amplitude of the voltage supplied to other devices in the matrix. More particularly, one embodiment of the present invention relates to a method and apparatus for selectively exciting a matrix of liquid crystals which comprise an optical display. The liquid crystal may be that disclosed in copending application of D. C. Green and W. R. Young entitled Nematic Materials and Method for Their Preparation," Ser. No. 299,991, filed Oct. 24, 1972, now U.S. Pat. No. 3,836,478'and assigned to the assignee of this application. The disclosure thereof is incorporated herein by reference.

It is generally useful, in controlling a matrix of voltage responsive devices, to be able to multiplex the driving signals to which the devices respond. To be able to perform this signal multiplexing, it is necessary that the voltage responsive devices not respond immediately to the absence of the driving voltage, but that their response to the absence of this voltage be delayed. Of course, the extent to which the driving signals can be multiplexed is dependent upon this delayed response which can be defined as a decay time (Y Another factor which controls the extent to which the driving signals can be multiplexed is the length of time during which a driving signal must be present for the device to respond thereto. This latter time can be referred to as a rise time (Y In determining the extent to which multiplexing can be utilized, or the size of a matrix which can be driven, the ratio (Y /Y of the decay time to the rise time is significant. The larger this ratio the larger the matrix that can be driven. A larger ratio means that for any given period of time during which a driving signal must be present, the period of time between driving signals can be enlarged. Of course, the period of time between driving signals for any element in the matrix can be utilized for driving other elements in the matrix.

Since the voltage responsive devices with which this invention is concerned are arranged in a matrix, it is convenient to drive them with row and column drive lines and multiplex the driving signals to drive one row at a time. The voltage applied to any given device in the matrix then is the difference between, the voltage applied on the column drive line with which the device is associated, and the voltage provided on the row drive line with which the device is associated. As a result, the contribution to the voltage provided to all devices in a given column, by the column drive line, is identical. Correspondingly, the contribution to the voltage difference at any device in a given row, by the row drive line is also identical. However, to drive some voltage responsive devices (examples of which are disclosed herein) it may be necessary to reduce the voltage at any selected device to zero without affecting the amplitude of the voltage delivered to any other device, including devices in rows and columns in common with the selected device. The method and apparatus for selectively exciting the voltage responsive devices described herein meets this criterion.

One of the characteristics of the nematic liquid crystals with which this invention is concerned is a dielectric anisotropy (Ae) which changes sign with frequency. That is, at some low frequency, f,, the dielectric anisotropy is positive (Ae 0) while at some higher frequency, f,,, the dielectric anisotropy is negative (Ae,, 0). See Fast Turn-Off Nematic Liquid Optical Devices of M. J. Freiser, U.S. Pat. application Ser. No. 359,824, filed May 14, 1973 now U.S. Pat. No. 3,857,629 and assigned to the assignee of the instant application. The disclosure of the foregoing application is incorporated herein by reference. Since the material of the crystal presents the highest dielectric constant possible to an applied electric field, it is possible, by controlling the voltage and frequency of one or more voltages applied across the device to control the orientation of the material. This control over the orientation of the material allows the material to be switched between a light transmissive state and a light extinguishing or opaque state. In order to increase the amount of driving signal multiplexing, the transmissive and light extinguishing states of the material are selected so as to increase the decay time (Y and to decrease the rise time (Y In other words, the states are so chosen that the material responds quickly to a driving signal and responds slowly to the absence of the driving signal. For purposes of the present description a driving signal may be any change in applied voltage which tends to order the material in such a configuration that it is light transmissive. In some instances the driving signal may actually be the removal of any voltage applied to the material where the applied voltage results in the material being light extinguishing.

SUMMARY OF THE INVENTION A matrix of voltage responsive devices are driven by a voltage difference provided between column and row drive lines, each of the devices being fed by one of a plurality of column drive lines and one of a plurality of row drive lines. The voltage difference normally provided at each of the devices are equal in magnitude and, in order to excite a selected device, the voltage across that device is reduced to zero without affecting the amplitude of the voltage across any of the other devices. Each of the column drive lines is normally provided with an alternating current voltage of identical phase. Each of the row drive lines is provided with an alternating current voltage equal in magnitude to the voltage provided to the column drive lines, but having its phase displaced by 21r/3 radians. In order to select any device the alternating current voltage provided to the column and row drive lines associated with that device is shifted in phase with respect to either of the voltages driving the other column or row drive lines by an amount equal to 21'r/3 radians. The effect of these driving voltages is to provide, at the selected device, a zero voltage difference between the column and row drive lines since these lines have voltages of equal amplitude and phase. The voltage difference at devices in the matrix, not associated with the row or column drive lines whose signal has been shifted in phase, remains unchanged. The phase of the voltage difference provided to nonselected devices associated with either the row or column drive lines whose phase has been shifted, changes, but the amplitude of the voltage difference provided to such devices is unchanging. As a result, only the selected device has its voltage difference set to zero without affecting the voltage difference provided to any of the other devices in the matrix.

In a number of preferred embodiments of the present invention, the voltage responsive devices comprise liquid crystals and, in particular, nematic liquid crystals. A suitable material for the nematic liquid crystal is disclosed in the aforementioned copending application of D. C. Green and W. R. Young, Ser. No. 299,991. This material has an index of refraction for any direction which varies and is related to the direction of the optic axis. Furthermore, the direction of the optic axis can be controlled by application of an electric field. The material also exhibits a dielectric anisotropy which changes sign as the frequency of the electric field is varied.

One type of optical display employing nematic liquid crystals is disclosed in the aforementioned copending application of Freiser, Ser. No. 359,824. In this type of device, the liquid crystal is enclosed in a flat-film configuration between parallel electrodes. The surface of the electrodes is suitably treated, as disclosed in the referred to application, such that the optic axis lies parallel to each of the electrodes. One of the electrodes, however, is rotated 90 with respect to the other so that in the quiescent state the optic axis of the liqiuid crystal rotates 90 from one electrode to the other. Therefore, with crossed polarizers adjacent each electrode, the combination of polarizers, electrode and liquid crystal, is transmissive in the quiescent state. With an electric field, applied perpendicular to the electrodes at a frequency giving a positive dielectric anisotropy the combination becomes light extinguishing. Therefore, if the entire matrix is subjected to this electric field which is then selectively removed, selective portions of the matrix will be light transmissive whereas the portions to which the electic field is applied will be light extinguishing. In this manner, information can be represented on a matrix of such devices.

In another embodiment of the present invention, in addition to the foregoing voltages applied to the matrix. a further alternating current voltage of higher frequency is also applied to the matrix. The nematic liquid crystal material exhibits a negative dielectric anisotropy atthe frequency of the further alternating current voltage. Furthermore, the column drive lines are not normally provided with this further driving voltage and all the row drive lines are normally provided with this further driving voltage of the equal amplitude and phase. In order to select any particular device, the column drive line associated with that device is provided with the further alternating current voltage with an amplitude twice that provided to the row drive lines, and at the same phase. The row drive line associated with the selecteddevice is provided with the further alternating current voltage at the same amplitude as that provided to all the other rows, but whose phase is displaced 11' radians. As a result, the selected device will have a voltage difference, at the higher frequency, three times that existing at any other selected device. The effect of this further driving voltage, in addition to the first driving voltage, is to further increase the decay time (Y and decrease the rise time (Y, of the'display. In this manner, a still larger matrix can be driven with multiplexed driving signals.

Other'combinations of driving signals can be provided to the matrix within the spirit and scope of the present invention.-

BRIEF DESCRIPTION OF THE DRAWINGS In the course of this description reference will be made to the accompanying drawings in which like reference characters identify identical apparatus and in which,

FIG. 1a illustrates a plan view of a nematic liquid crystal matrix comprising an optical display;

FIG. 1b is a cross section of FIG. 1a taken on lines lb-lb;

FIG. 2 is a representation of one element of the matrix with associated light polarizers;

.FIG. 3 is a representation of the variation of the dielectric anisotropy of the nematic liquid crystal with frequency;

FIG. 4 is a diagram in voltage space illustrating the operating points of the nematic liquid crystal;

FIGS. 5a through 5d illustrate voltages applied to column and row drive lines and resulting voltage differences at each device in a matrix of devices;

FIGS. 6a, through 6d illustrate the voltages applied to a matrix of voltage responsive devices and the resulting voltage differences;

FIG. 7 illustrates the pattern resulting from the application of such voltages;

FIG. 8 shows a suitable column drive generator;

FIG. 9 shows a suitable row drive generator; and

FIGS. 10a andlOb show suitable column and row drivers.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1a illustrates a matrix of voltage responsive devices in accordance with one preferred embodiment of the present invention. In particular, a plurality of orthogonal transparent electrodes 14 and 16 are provided. Column electrodes 16 overlie and are perpendicular to row electrodes 14. FIG. 1b illustrates a cross section of the matrix illustrated in FIG. 1a taken on lines lblb. In addition to the electrodes 16 and 14 illustrated in FIG. 1b, polarizers and 12 are also illustrated. Furthermore, the nematic liquid crystal material 18 is illustrated as lying between the orthogonal electrodes 16 and 14. The polarizers l0 and 12 are parallel to one another and one of the polarizers has its polarization axis rotated 90 with respect to the axis of the other polarizer. The surfaces of the electrodes 14 and 16 which are in contact with the nematic liquid crystal 18 are treated such that the molecules of the liquid crystal, and therefore the optic axis of the material, lie parallel to the surface of the electrodes in a preferred direction. The preferred direction at the electrodes 16 is rotated 90 with respect to the preferred direction at the electrodes14. In the quiescent state, therefore, the optic axis of the nematic liquid crystal 18 willbe parallel to the electrodes 14in the region adjacent the electrodes 14, and will be parallel to the electrodes 16 adjacent the region of electrodes 16. Since these preferred directions are rotated 90 one with respect to the other, the optic axis of the nematic liquid crystal 18 rotates 90 from the region at electrodes 14 to the region at electrodes 16. As a result the optic axis twists between the two electrodes.

FIG. 2 illustrates one element of the matrix illustrated in FIG. 1a with the electrodes 14 and 16 enclosing the nematic, liquid crystal 18 in a flat-film configuration. Adjacent each of the electrodes 14 and 16 are polarizers 10 and 12. In the absence of an electric field, the optic axis of the nematic liquid crystal 18 rotates 90 from the region adjacent electrode ,16 to the region adjacent electrode 14. Therefore, with crossed polarizers l0 and 12, as illustrated in FIG. 2, the combination of polarizers 10 and 12, transparent electrodes 16 and 14, and nematic liquid crystal 18, will be light transmissive. That is, although entering light is polarized by polarizer 10 in a particular direction, the rotation of the optical axis .will result in the light emitted from electrode 14 being polarized in accordance with the polarization of the polarizer 12. As a result, the cell is transmissive. No light source isillustrated although those skilled in the art will understand how to provide a suitable source. I

FIG. 3 illustratesthe relationship of the dielectric anisotropy of the'-liquid crystal material with respect to frequency and also illustrates the variation in e and L As shown in FIG. 3,6" at low frequencies, is

' greater than 221 Recalling that the dielectric anisotropy As is defined as Ae=e e it is apparent that the material, at DC and low frequencies, exhibits positive dielectric anisotropy. Above a frequency f, at which 6 =e the material exhibits negative dielectric anisotropy. Since a material subjected to an electric field will align itself, if possible, in the direction which presents the highest dielectric constant to the electric field, it is apparent that with electric fields at frequencies below f, the molecules of the nematic material 18 will tend to align themselvesparallel with the electric field. As a result the cell will be light extinguis'hing or opaque. To the contrary, however, when electric fields are applied with a frequency greater than f, the molecules of the nematic material will tend'to align themselves perpendicular to the electric field and thus the cell will be light transmissive.

With material having characteristics such as those represented in FIG. 3, we can then represent the effect on the material of a combination of electric fields of low and high frequency. If we assume that the low frequency f and high frequency f,, arechosen such that the absolute value of the dielectric anisotropy is equal at these two frequencies |Ae I A6,. I), we can construct a phase diagram in voltage space with the ordinate representing the magnitude of the high frequency voltage and the abscissa representing the magnitude of the low frequency voltage. In this voltage space representation the phase boundaries are at 45 to the axis. The phase boundary intersects the abscissa at a value v, which is the minimum voltage necessary to cause the molecules of the nematic liquid crystal 18 to align themselves parallel to the electric field. The phase boundaries define two regions, the first, denoted IIII, in which the molecules of the nematic liquid material are parallel with the electric field, and the second, denoted I, where the molecules remain perpendicular to the applied electric field. With crossed polarizers such as that illustrated in FIG. 2, of course, the twisted state (I) will be light transmissive and the parallel state (IIII) will be light extinguishing. It is within the scope of the present invention to employ parallel polarizers such that the parallel state (IIII) is light transmitting and the twisted state (I) is light extinguishing.

As has been explained above, information is written into the matrix one row at a time. There are two significant factors in determining the number of rows that can be written, or the extent to which the information signals can be multiplexed into the matrix. The first significant factor is the time (Y taken by the elemental portion of the nematic liquid crystal to respond to the application of a signal. The second significant factor is the decay time (Y,,,), the time taken by an elemental portion of the nematic liquid crystal material to return from an abnormal to its normal state. As in information theory, the normal state of an element in a matrix conveys no information whereas the abnormal state conveys the information. Thus, for instance, if the element is normally in the state B shown in FIG. 4, and it was at times switched to state A (see FIG. 4), state B would be the off or normal state and state A would be the on or abnormal state. Alternatively, the cell could be in statelC (FIG. 4) as the normal state and it could, at times, beswitched to state D (FIG. 4) for purposes of displaying information. Both of these state combinations represent preferred embodiments of the present invention taken with the crossed polarizers as illustrated in FIG. 2. For purposes of this description we will describe first the general case, i.e., the transition from state C to state D, and then discuss the transition from state B to state A, as a special case.

Referring to FIG. 4, the coordinates of state C (V V and D(O, 3V correspond to the off and on states respectively. It can be shown that the characteristic time for the response of the liquid crystal which is initially at C to a voltage pulse driving it to state D is In the foregoing, y is a viscosity, L is the thickness of the cell and V is the critical voltage, which is defined as the low frequency voltage required to overcome surface forces in driving the cell from the twisted state to the parallel state in the absence of a high frequency electric field. In returning from state D to state C the characteristic response time of the liquid is The ratio of these times, that is the ratio of Y /Y, is a measure of the extent to which signals driving the matrix can be multiplexed and In order to maximize this ratio it is desirable to operate as close to the transition region as feasible (for the state C) and with as large a high frequency voltage as possible. For purposes of this description, state C (V V lies close to the transition region and state D (0, 3V has a high frequency voltage three times that of state C and a zero low frequency voltage.

The function to be performed by the information writing equipment is to selectively change the state of a selected element from state C in which it is light extinguishing to state D in which it is light transmissive for those portions of the display which are to be illuminated. The high frequency drive will be of the form KV sin (W NH) where N equals zero or I and K l or 2, or O. In describing the driving voltages applied to the orthogonal conductors, we will describe the voltages necessary to reproduce the pattern illustrated in FIG. 7. FIG. 7 illustrates a three by three matrix in which one element in each row is illuminated. Of course, those with ordinary skill in the art will understand that more than one element in each row may be illuminated and in some rows no elements need be illuminated. The pattern illustrated in FIG. 7 is thus arbitrary and merely for purposes of explanation. Each of the elements in the matrix, 21 through 29, is associated with one row and one column conductor or drive line. The voltage applied across each element of the matrix is the difference between the voltage applied on the column conductor associated with the element and the voltage applied on the row conductor associated with the element. Looking at FIGS. 4 and 7 it is apparent that for elements 21, 23, 24, 25, 27, and 29 to be extinguished, the high frequency voltage should be V and that the high frequency voltage on illuminated elements 22, 26, and 28 should be at some point in the writing cycle 3V The manner in which this is effected will now be explained with reference to FIGS. 6a through 6d.

FIG. 6a illustrates the exemplary matrix in the condition before writing information therein. Since each of the elements in the matrix is to be light extinguishing it will be in state C (FIG. 4) with a high frequency voltage difference of V To effect this, each of the columns is driven with a zero voltage of frequency f (K The first row, that is, the row comprising matrix elements 21 through 23 has applied to it a voltage with amplitude V with a zero relative phase shift (K 1, N

= 0). As has been previously explained the voltage difference across each element in the matrix is the difference between the voltage applied to it by the column drive line and the voltage applied to it by the row drive line. Therefore, each of the elements 21 through 23 will have a voltage of magnitude V with a relative phase angle ofrr radians. Each of the other rows in the matrix has the same voltage applied to it and therefore each of the other elements in the matrix has the same resulting potential difference across it. This is schematically illustrated in FIG. 6a wherein each matrix element has the numerals V /1r therein indicating a voltage magnitude of V at a relative phase angle of 1r radians.

FIG. 6b illustrates the state of the matrix upon writing information in the first row. In particular, the column drive lines have voltages of zero, 2V and zero at a relative phase angle of zero, zero, and zero, respectively.

The first row drive line is driven with a voltage at a magnitude of V with a relative phase angle of 11' radians (N I). As a result, matrix element 21 has a potential difference of magnitude V and a relative phase angle of zero. Matrix element 22, however, has a resulting potential difference of 3V at a zero phase angle. Matrix element 23 has the same potential difference as matrix element 21. The other two rows in the matrix are driven at a voltage of magnitude V at a zero relative phase angle. As a result, matrix elements 24, 26, 27, and 29 have a potential difference across them of magnitude V at relative phase angle 1r radians. The matrix elements 25 and 28 have the same magnitude potential difference but a zero relative phase angle. It will be seen that the high frequency component across matrix element 22 is three times the high frequency voltage across any other matrix element.

FIG. 6c illustrates the condition of the matrix when writing information in the second row. The column drive line voltages applied to the matrix are zero, zero, and 2V at a relative phase angle of zero radians. The row drive lines have applied to them respectively V at a zero phase angle, V H at a phase angle of 11 radians and V at a zero phase angle. As a result, matrix elements 21, 22, 27 and 28 have a voltage difference of magnitude of V and a relative phase angle of w radians. Matrix elements 23, 24, 25, and 29 have a voltage difference of V at a zero phase angle. Matrix element 26 has a voltage difference magnitude 3V H at a zero phase angle. Thus, it will be seen that the high frequency voltage across matrix element 26 is three times the high frequency voltage across any other matrix element.

Now let us refer to FIG. 6d which shows writing in the third row of the matrix. The column drive lines have applied thereto, respectively, a zero voltage, a voltage of 2V at a zero relative phase angle, and zero voltage. The row drive lines have applied to them voltages of V H at a zero phase angle, V at a zero phase angle, and V at a relative phase angle of 11' radians. The resulting difference across matrix elements 21, 23, 24 and 26 is V at a relative phase angle of w radians. The voltage difference across matrix elements 22, 25, 27 and 29 is V at a zero phase angle. The voltage difference across matrix element 28 is 3V at a zero phase angle.

In the foregoing manner the high frequency component of the voltages necessary to write the pattern illustrated in FIG. 7 have been applied.

Referring again to FIG. 4 we see that in addition to increasing three fold the high frequency voltage across the selected elements, it is also necessary, simultaneously, to reduce the low frequency voltage at those selected elements to zero in order to effect the transition from state C to state D. The manner in which the low frequency voltages are controlled will now be explained with reference to FIGS. 5a through 5d.

The low frequency voltages applied to the row and column drive lines are of the form V sin (W M2 1r /3) where M =l, or 1. FIG. a illustrates the condition of the row and column drive lines'and the condition of each of the matrix elements in the state when no information is being written. The voltages applied at each of the column drive lines are of a magnitude V with a relative phase angle of zero, that is, M O. V V VS. The voltages applied to each of the row drive lines is of magnitude V and a relative phase angle of 2 1r/3, that is M 1. As a result the low frequency voltage difference applied to each of the matrix elements has a magnitude V with a relative phase angle of 'rr/6. Since the voltage difference applied to each of the matrix elements is the difference between the voltage on the column drive line and the row drive line, constructing a vector addition of V V? at an angle of zero with the vector V,/ V?) at an angle of -2 'rr/3 results in a vector of magnitude V at an angle 1r/6. Thus, each of the matrix elements has a-low frequency voltage applied thereto of magnitude V FIG. 5b illustrates the condition of the matrix when writing information in the first row. The voltage magnitudes applied to each of the row and column drive lines are identical and are each equal to V V, VS. The relative phase angle of the voltages applied to the column drive lines are zero, 2 'rr/3 and zero corresponding to M O, -l and 0. The relative phase angles of the voltages applied to the row drive lines are 2 1r/3, 2 77/3 and 2 1r/3, respectively. As a result, the voltage across matrix elements 21 and 23 has a magnitude of V and a relative phase angle of 1-r/6 radians. The voltage applied to matrix elements 24, 26, 27, and 29 has a magnitude V at a phase angle of-1r/6. The voltage applied to matrix elements and 28 has a magnitude V and a relative phase angle 'rr/2. Finally, matrix element 22 has a zero low frequency voltage applied thereto. Referring back now to FIG. 6b, it will be seen that matrix element 22 has a zero low frequency voltage applied thereto and a high frequeny voltage of 3V whereas each of the other elements in the matrix has a high frequency voltage applied thereto of V and a low frequency voltage applied thereto of V Thus referring now to FIG. 4 it will be seen that matrix element 22 has been switched from state C to state D.

Refer now to FIG. 5c which illustrates writing in the second row. The voltages applied to the column drives are V at a zero relative phase angle, V at a zero relative phase angle, and V at a phase angle of-2 11/3 radians. The voltages applied to the row drive lines are each V, and have phase angles of +2 17/3, -2 1r/3, and +2 1r/3, respectively. The resulting voltage difference at elements 21, 22, 27, and 28 is a voltage with amplitude V at a relative phase angle of 7r/6. The resulting voltage difference at matrix elements 24 and 25 has an amplitude of V with a relative phase angle of 1r/6. The voltage difference at elements 23 and 29 has an amplitude of V, at a relative phase angle 1r/2. The voltage difference at matrix element 26 is zero. Again, referring to FIG. 6c it will be seen that each of the elements in the matrix other than element 26 has the same high frequency voltage V and the same low frequency volt-' age V related to state C (FIG. 4). However, matrix element 26 has a zero low frequency voltage and a high frequency voltage of 3V corresponding to state D (FIG. 4). Thus, when the voltages of FIGS. 5c and 6c are simultaneously applied, matrix element 26 is switched from state C to state D.

Referring now to FIG. 5d, this illustrates writing in the third row. The voltages applied to the matrix are illustrated as well as the resulting low frequency voltage across each of the matrix elements. A detailed discussion is not believed necessary in view of the discussions with respect to FIGS. 5b and 5c. Matrix element 28 has a zero low frequency voltage across it. All the other matrix elements have a low frequency voltage of magnitude V Referring to FIG. 6d, it will be seen that the high frequency voltage across matrix element 28 is 3V H whereas the high frequency voltage across each of the other matrix elements is V Thus, with the voltages of FIGS. 6d and 5d applied to the matrix, all the elements in the matrix will be in state C except for matrix element 28 which will be in state D.

Reference to FIGS. 5a through 5d illustrates that the magnitude of the low frequency voltage driving all the columns and all the rows remains unchanging and furthermore that all the columns are normally driven with a voltage of zero relative phase angle. Likewise, all the rows are normally driven with a relative phase angle of +2 7r/3.I-Iowever, when it is desired to select a particular device, the relative phase angle of the voltages on the row and column drive lines associated with that device are shifted to a relative phase angle of 2 1r/3. Those with ordinary skill in the art will understand that the absolute phase shift values are a matter of choice; what is significant is the relative phase shift between the unselected columns and unselected rows and between the selected columns and rows.

In order for each selected element to make the transition the voltage patterns show in any of FIGS. 5a-6a; 5b-6b, 5c-6c and 5c-6d, must be maintained for a sufficient period of time for the nematic liquid crystal material to respond thereto. This time has been defined as Y above. Furthermore, in order for the pattern to remain apparently stationary to a viewer, the time elapsed between applying the voltage patterns shown in FIGS. 5b-6b for instance, and the time the same pattern is repeated must be less than the decay time (Y,,,) for the nematic liquid crystal material which has also been defined above.

When it is desired to change the information being displayed on a matrix, such as that shown in FIG. 7, it is only necessary to cease writing the information. After a time equal to Y the pattern will disappear. If rapid erasure is desired for the entire panel a short interruption in the high frequency voltage drive on all lines, rows and columns, will rapidly drive the entire matrix to a light extinguishing condition.

The foregoing apparatus has been operated with the material disclosed in the aforementioned Green et al application with a cell /2 mil in thickness. For this material V equals 3 volts and the transition region, shown as shaded in FIG. 4, has a width of approximately 2 volts. The low frequency was chosen as hertz and the high frequency was chosen as 2 kilohertz with V equal to 20 volts and V equal to 27 volts (RMS). The cycle time used was I second with 25 millisecond writing pulses. With this material and the driving arrangement disclosed herein, it has been projected that a matrix of up to 80 rows may be written.

FIGS. 6a through 6d illustrate that there are three high frequency voltages that are required to operate the matrix, i.e., a voltage of 2V a voltage of V at the same phase as the voltage 2V and a third voltage V shifted by 1r radians with respect to the other two high frequency voltages. FIG. 8 illustrates an apparatus which may be used to generate these voltages. A high frequency voltage generator 31 of frequency f feeds a push-pull amplifier 32. One output of push-pull amplifier 32 is coupled to terminal 33 where the voltage 2V is available. A voltage divider, comprising resistors 36 and 37 of equal magnitude, is serially connected between ground and terminal 33. Thus, the junction of resistors 36 and 37, i.e., terminal 34, provides a voltage V at the same phase angle as the voltage 2V The other output from push-pull amplifier 32 is connected to a resistor 39 which is connected to resistor 38 which, in turn, is grounded. Resistors 39 and 38 are of equal magnitude. A terminal, 35, at the junction of resistors 39 and 38 provides the voltage V which is shifted by 11 radians from the other high frequency voltages available at taps 33 and 34.

Reference to FIG. a through FIG. 5d illustrates that there are three low frequency voltages required. To provide these voltages, a voltage generator 41 (FIG. 9) operates at a frequency f The output of generator 41 is coupled to terminal 42 to provide the voltage V. In order to provide a voltage shifted by 2 17/3 radians from this voltage a phase shifter 45 is connected to the output of voltage generator 41. The output of phase shifter 45 is connected to terminal 43 to provide a voltage V shifted in phase with respect to the voltage at terminal 42 by 2 1r/3 radians. In a like manner, the voltage V/2 1r/ 3 is available at terminal 44 from a phase shifter 46. Alternatively a three-phase source may be used.

In order to make use of these voltages V and V, they must be selectively connected to the column and row drive lines. FIGS. 5a through 5d illustrate that the column drive lines utilize only two low frequency voltages, that is, they require V'/0 and V'/2 1r/3. FIG. a illustrates a typical column driver which has two inputs 51 and 52. Input 51 is the low frequency input and it is connected to switching contact 53 which is selectively positionable in two positions. In a first position it connects to contact 42 to provide the voltage V'/0. In its other position it contacts terminal 44 to provide the voltage V'/2 17/3. Input 52 to column driver 50 is the high frequency input and it is likewise connected to a switching contact 54 which is capable of assuming two different positions. In a first position it connects with terminal 33 to provide the voltage ZV /O. In its second position, it contacts terminal 55 to provide a zero voltage since terminal 55 is grounded. FIGS. 6a through 6d illustrate that the column drive lines need noly these two voltages.

FIG. 10b illustrates a typical row driver 60. This driver 60 also has two inputs 61 and 62. The low frequency input 61 is connected to a switching contact 63 capable of assuming two positions. In a first position it makes contact with terminal 43 to provide the voltage V/2 1r/3. In its second position, it contacts terminal 44 to provide the voltage V/2 7r/3. The high frequency input to row driver 60 is via input 62 which is connected to a switching contact 64. Contact 64 is capable of assuming two positions. In a first position it contacts terminal 34 to provide the voltage V /O. In its second position, it. contacts terminal 35 to provide the voltage VH/7T- The switching contacts 53, 54, 63, and 64 may be manually operable. However, in a preferred embodiment these switching contacts represent electronic switching circuits which can respond much more quickly than can the manual switches to make the different connections required. The particular form of electronic switching arrangement forms no part of the present invention as there are many varieties in the prior art which can be employed for this function.

In another embodiment of the present invention the light extinguishing the light transmissive states of the material are taken at state B and A. respectively. To effect this, it is only necessary to remove all high frequency voltages, that is, V equals zero continuously. Furthermore, since the abscissa at state B may be less than V,, the low frequency voltage must be correspondingly reduced. The same phase changes are made as is illustrated in FIGS. 5a through 5d to produce the pattern shown in FIG. 7. As has been explained above, the ratio of the decay time to the rise time, Y /Y, is a measure of the size of a matrix which can be written with an apparently stationary pattern. In the case where there is no high frequency voltage that expression reduces to Since V is a characteristic of the nematic liquid crystal material utilized, this value will, of course, remain unchanged; However, since the abscissa value for state B is less than the value V utilized in the first embodiment illustrated, this ratio for the BA operation will be less than the ratio for the C-D combination. As a result, the limitingmatrix size which can be controlled employing the C-D states will be larger than the matrix which can be controlled .using the BA states.

In another preferred embodiment of the present invention the nematic liquid crystal material can be driven between the states EF (FIG. 4). To maintain similarly high values for the ratio Y /Y it is preferred to maintain the polarizers 16 and 18 in a parallel configuration such that state F would correspond to a light extinguishing state and state E would correspond to a light transmitting state.

Furthermore the state combination A-E could also be used. Since the low frequency voltage of state B and state E may be varied (so long as it is above V this combination is similar to the BA combination with the exception that the off and on states have been changed by making the polarizers parallel instead of orthogonal. Another state combination that could be used with parallel polarizers is F-E. In this case the phase shift arrangement of FIGS. 5a-5d is used to control the matrix.

What is claimed is:

1. Apparatus for selectively controlling a matrix of voltage responsive devices in which each device is associated with and driven by a different pair of a plurality of row and column drive lines, each said device responding to a difference in voltage between its associated column and row drive line, the improvement comprising drive means for normally providing a voltage to each said device equal in amplitude to a voltage supplied to each other device and for selectively reducing the voltage at a selected device to zero without varying the voltage amplitude supplied to each other device, said drive means including,

means providing three alternating current voltages of the form V sin'( wt m2 IT/3) wherein each of said voltages has a different value of m selected from the group comprising l, 0, +1, and wherein V V3 times said voltage and,

switching means normally connecting one of said voltages to said column drive lines, and another of said voltages to said row drive lines and for selectively connecting said third voltage only to a row and column drive line associated with said selected device,

whereby a voltage normally exists across each of said devices equal in amplitude to the voltage across each other said device and, at times, a zero voltage exists across a selected device while said equal voltage amplitude is maintained across all said other devices.

2. The apparatus of claim 1 in which said matrix comprises an optical display including means for providing light to said display, each of said devices comprises a fiat-film nematic liquid crystal cell, with a positive dielectric anistropy at the frequency of said voltage.

3. The apparatus of claim 2 which further includes a first and second light polarizing means, said cell being located between said first and second light polarizing means.

4. The apparatus of claim 3 in which said first light polarizing means is oriented with a polarizing axis orthogonal to a polarizing axis of said second light polarizing means.

5. The apparatus of claim 4 in which said voltage is greater than the critical voltage of said nematic liquid crystal.

6. The apparatus of claim 5 in which said voltage exceeds said critical voltage by at least two volts.

7. The apparatus of claim 6 which further includes means for coupling voltages at a second frequency to said drive lines,

said nematic liquid crystal having a negative dielectric anistropy at said second frequency.

8. The apparatus of claim 7 in which said means for coupling normally provides no voltage at said second frequency to said column drive lines and normally provides a second voltage of equal magnitude and phase at said second frequency to each of said row drive lines and, at said times, provides to a column drive line associated with said selected device of voltage at said second frequency greater in amplitude than said second voltage at the same phase as said second voltage, and provides to a row drive line associated with said selected device a voltage at said second frequency equal in amplitude to said second voltage but displaced 1r radians relative to said second voltage.

9. The apparatus of claim 8 in which said voltage at said second frequency greater in amplitude is twice said second voltage in amplitude.

10. The apparatus of claim 1 in which said matrix comprises an optical display including means for providing light to said display, each of said devices comprises a flat-film nematic liquid crystal cell with a negative dielectric anistropy at the frequency of said voltage.

11. The apparatus of claim 10 which further includes a first and second light polarizing means, said cell being located between said first and second light polarizing means.

12. The apparatus of claim 11 in which said first light polarizing means is oriented with a polarizing axis parallel to a polarizing axis of said second light polarizing means.

13. The apparatus of claim 12 which further includes means for coupling voltages at a second frequency to said drive lines,

said nematic liquid crystal being a positive dielectric anistorpy at said voltages of second frequency. 14. The apparatus of claim 13 in which said means for coupling normally provides no voltage at said second frequency to said column drive lines and normally provides a second voltage of equal magnitude and phase at each of said row drive lines and, at times, provides to a column drive line associated with said selected device a voltage at said second frequency greater in amplitude than said second voltage, and at the same phase as said second voltage, and provides to a row drive line associated with said selected device a voltage at said second frequency equal in amplitude to said second voltage but displaced 1r radians relative to said second voltage.

15. The apparatus of claim 14 in which said voltage at said second frequency of greater amplitude is twice said second voltage in amplitude.

16. A method of controlling a matrix of voltage responsive devices in which each of said devices is connected between one row and one column drive line of a plurality of row and column drive lines, for normally supplying an equal amplitude voltage to each said device and for, at times, supplying a zero amplitude voltage to any selected device while continuing to supply said equal amplitude voltage to all said other devices, said method comprising the steps of,

providing three voltages, each of the form V sin (wt m2 7r/3) wherein each of said voltages has a different value of m selected from the group comprising l, 0, +1

coupling one of said voltages to all said column drive lines and another of said voltages of said row drive lines,

and at times, coupling, on row and column drive lines associated with a selected device said third voltage and removing said coupling to said another and one of said voltages respectively,

whereby a voltage normally exists across each of said devices which is equal in amplitude to a voltage existing across all other said devices and at times a voltage across said selected device is reduced to zero without affecting the voltage amplitude across all said other devices. 

1. Apparatus for selectively controlling a matrix of voltage responsive devices in which each device is associated with and driven by a different pair of a plurality of row and column drive lines, each said device responding to a difference in voltage between its associated column and row drive line, the improvement comprising drive means for normally providing a voltage to each said device equal in amplitude to a voltage supplied to each other device and for selectively reducing the voltage at a selected device to zero without varying the voltage amplitude supplied to each other device, said drive means including, means providing three alternating current voltages of the form V sin ( wt + m2 pi /3) wherein each of said voltages has a different value of m selected from the group comprising -1, 0, +1, and wherein V square root 3 times said voltage and, switching means normally connecting one of said voltages to said column drive lines, and another of said voltages to said row drive lines and for selectively connecting said third voltage only to a row and column drive line associated with said selected device, whereby a voltage normally exists across each of said devices equal in amplitude to the voltage across each other said device and, at times, a zero voltage exists across a selected device while said equal voltage amplitude is maintained across all said other devices.
 2. The apparatus of claim 1 in which said matrix comprises an optical display including means for providing light to said display, each of said devices comprises a flat-film nematic liquid crystal cell, with a positive dielectric anistropy at the frequency of said voltage.
 3. The apparatus of claim 2 which further includes a first and second light polarizing means, said cell being located between said first and sEcond light polarizing means.
 4. The apparatus of claim 3 in which said first light polarizing means is oriented with a polarizing axis orthogonal to a polarizing axis of said second light polarizing means.
 5. The apparatus of claim 4 in which said voltage is greater than the critical voltage of said nematic liquid crystal.
 6. The apparatus of claim 5 in which said voltage exceeds said critical voltage by at least two volts.
 7. The apparatus of claim 6 which further includes means for coupling voltages at a second frequency to said drive lines, said nematic liquid crystal having a negative dielectric anistropy at said second frequency.
 8. The apparatus of claim 7 in which said means for coupling normally provides no voltage at said second frequency to said column drive lines and normally provides a second voltage of equal magnitude and phase at said second frequency to each of said row drive lines and, at said times, provides to a column drive line associated with said selected device of voltage at said second frequency greater in amplitude than said second voltage at the same phase as said second voltage, and provides to a row drive line associated with said selected device a voltage at said second frequency equal in amplitude to said second voltage but displaced pi radians relative to said second voltage.
 9. The apparatus of claim 8 in which said voltage at said second frequency greater in amplitude is twice said second voltage in amplitude.
 10. The apparatus of claim 1 in which said matrix comprises an optical display including means for providing light to said display, each of said devices comprises a flat-film nematic liquid crystal cell with a negative dielectric anistropy at the frequency of said voltage.
 11. The apparatus of claim 10 which further includes a first and second light polarizing means, said cell being located between said first and second light polarizing means.
 12. The apparatus of claim 11 in which said first light polarizing means is oriented with a polarizing axis parallel to a polarizing axis of said second light polarizing means.
 13. The apparatus of claim 12 which further includes means for coupling voltages at a second frequency to said drive lines, said nematic liquid crystal being a positive dielectric anistorpy at said voltages of second frequency.
 14. The apparatus of claim 13 in which said means for coupling normally provides no voltage at said second frequency to said column drive lines and normally provides a second voltage of equal magnitude and phase at each of said row drive lines and, at times, provides to a column drive line associated with said selected device a voltage at said second frequency greater in amplitude than said second voltage, and at the same phase as said second voltage, and provides to a row drive line associated with said selected device a voltage at said second frequency equal in amplitude to said second voltage but displaced pi radians relative to said second voltage.
 15. The apparatus of claim 14 in which said voltage at said second frequency of greater amplitude is twice said second voltage in amplitude.
 16. A method of controlling a matrix of voltage responsive devices in which each of said devices is connected between one row and one column drive line of a plurality of row and column drive lines, for normally supplying an equal amplitude voltage to each said device and for, at times, supplying a zero amplitude voltage to any selected device while continuing to supply said equal amplitude voltage to all said other devices, said method comprising the steps of, providing three voltages, each of the form V sin (wt + m2 pi /3) wherein each of said voltages has a different value of m selected from the group comprising -1, 0, +1, coupling one of said voltages to all said column drive lines and another of said voltages of said row drive lines, and at times, coupling, on row and column drive lines associated wiTh a selected device said third voltage and removing said coupling to said another and one of said voltages respectively, whereby a voltage normally exists across each of said devices which is equal in amplitude to a voltage existing across all other said devices and at times a voltage across said selected device is reduced to zero without affecting the voltage amplitude across all said other devices. 